Rework the feature overrides logic so all feature defines go into a
header rather than the command line. This has a few benefits. Most
notably, we can reuse the header in the gyp build. Another nice result
is that changes to the configuration automatically result in a full
rebuild and Debug and Release builds can have different configurations.

All feature define processing happens now during the configuration phase,
instead of when running autogen.sh. This means that we need to distribute
the feature processing script.

* GNUmakefile.am: Read feature defines from WebKitFeatures.txt when kicking
off a build. Also include generate-feature-defines-files in the distribution.
* Source/autotools/SetupAutoconfHeader.m4: Remove an extra ENABLE_3D_RENDERING
from the autoconf header, as it is already provided by the feature configuration.
* Source/autotools/SetupWebKitFeatures.m4: Added.
* Source/autotools/webkitfeature.m4: Removed. The functionality has been moved
to SetupWebKitFeatures.m4.
* autogen.sh: Remove the call to override-feature-defines.
* configure.ac: Now include SetupWebKitFeatures.

Source/WebCore:

* GNUmakefile.am: No longer include GNUmakefile.features.am. We don't
have to process the result of this file or do any modification of
feature_defines_override, because this variable is gone.

Tools:

* Scripts/webkitdirs.pm:
(runAutogenForAutotoolsProjectIfNecessary): Rename the feature file to
WebKitFeatureOverrides.txt to better match the other filenames used in
the new feature processing.
* gtk/generate-feature-defines-files: Added. A script which takes as an argument
the default features, overrides them with the contents of WebKitFeatureOverrrides.txt
and writes WebKitFeatures.txt and WebKitFeatures.h.
* gtk/override-feature-defines: Removed.